(1) Field of the Invention
This invention relates to a process of vapor phase epitaxy, which process is able to provide a good grown layer of a compound semiconductor.
(2) Description of the Prior Art
As processes for epitaxially growing compound semiconductors of Groups III-V metals, there are the liquid phase epitaxy and vapor phase epitaxy. The latter epitaxy is suited for mass production. Among various vapor phase epitaxial growth processes, the MOCVD process (Metal-Organic-CVD process) is especially useful as it permits to control the vapor phase and solid phase substantially at the same proportions and features superior controllability to the halide process.
A conventional MOCVD process is illustrated in FIG. 1, in which end caps 2,3 are provided respectively at both ends of a reactor tube 1 and gas-feeding tubes 4,5 are provided through the end cap 2. When growing InP films, for example, a metal alkyl which serves a source for In such as TEI (triethylindium) and PH.sub.3 gas which yields P are supplied into the reactor tube 1, independently through their corresponding gas-feeding tubes 4,5.
The gas, which has flown out of the reactor tube 1, is then exhausted through an outlet 6. A susceptor 8, which lies on a boat 7, is made of SiC-coated graphite. On the susceptor 8, an InP substrate 9 is mounted. The substrate 9 is heated inductively using an rf-coil 10. The temperature of the substrate 9 is detected by a thermo-couple 11, by which the rf-coil 10 is usually feed-back controlled so as to control the temperature of the substrate 9 at a constant level. When carrying out for example an epitaxial growth of InP on the InP substrate 9 in the above apparatus, the growth rate is governed generally by the amounts of source gases containing Group-III elements. Where no external heater 12 is provided with the chamber 1 in the apparatus of FIG. 1, the growth rate remains very slow even if the flow rate of TEI which is a source material for In, the flow rate of PH.sub.3 and the growth temperature are varied. When, for example, TEI is placed in a constant-temperature tank of 650.degree. C. and is bubbled with 100 cc/min. of H.sub.2 and 200 cc/min. of an H.sub.2 -base gas containing 2% of PH.sub.3 is fed together with 1 l/min. of an H.sub.2 carrier gas and the epitaxial growth process is carried out for 90 minutes at a growth temperature of 650.degree. C., an epitaxial layer of InP is formed on the substrate 9 to a thickness of about 0.1 .mu.m. Accordingly, it is impractical to use the apparatus of FIG. 1 as the growth rate is too slow (Where the heater 12 is not provided).
This slow growth rate can be attributed to the fact that the thermal decomposition of PH.sub.3 does not proceed well and PH.sub.3 is not decomposed and undergoes reactions with TEI in the course of its flow toward the substrate 9 to form complexes such as (C.sub.2 H.sub.5)3.PH.sub.3 and the like, whereby impairing the growth of InP. In the apparatus of FIG. 1, there is another inconvenience that these complexes deposit on the inner walls of the reactor tube 1 and cap 2.
With a view toward solving the above inconvenience, it has also been proposed to provide a heater 12 on the gas-feeding tube 5 which is employed as the PH.sub.3 gas line as illustrated in FIG. 1 so that PH.sub.3 gas in preheated to 800.degree. C. or so and the decomposition of PH.sub.3 gas is thus promoted. This process is able to improve the growth rate to a certain extent but causes the growth of InP to occur on the end cap 2 and the reactor tube 1 besides the substrate 9. Therefore, the efficiency of growth of InP on the substrate 9 is lowered. Unless the heater 12 is disposed very close to the reactor tube 1, a great deal of P is allowed to deposit on the wall of the gas-feeding tube 5 between the heater 12 and reactor tube 1 and the efficiency is hence lowered further. If the heater 12 is incorporated, it is necessary to construct the portion of the gas-feeding tube 5, which portion is surrounded by the heater 12, with a siliceous material, thereby tending to make the apparatus complex. If P is caused to deposit in a large amount on the wall of the tube and the like, the operation of the apparatus is hampered. Thus, it is extremely inconvenient to allow P to deposit in a large amount on the inner walls of the apparatus.